Today decides the length.
When daylight lasts twelve civil hours, a daylight Seasonal Hour also lasts sixty minutes. On a long summer day it becomes longer. On a short winter day it becomes shorter.
The clock follows sunrise and sunset.
The first daylight hour begins at sunrise. The twelfth ends at sunset. The night then receives its own twelve equal divisions until the next sunrise.
Why Roman numerals?
SOLAR uses a distinct notation so a Seasonal Hour cannot be mistaken for Local or Solar Time. A reading such as XI · 15 means fifteen minutes into the eleventh Seasonal Hour.
What happens near the poles?
When a date has no sunrise or no sunset, there is no complete daylight or night interval to divide in the normal way. SOLAR reports that the Seasonal Hour is unavailable rather than inventing a value.
What the mode teaches
A fixed sixty-minute hour makes the year feel uniform. Seasonal Hours reveal how dramatically the amount of light actually changes.